1. Field of the Invention
The invention in general relates to magnetic detectors, and particularly to a method and apparatus for nulling the detector in the presence of the earth's magnetic field at the detector location.
2. Description of the Prior Art
Magnetic detectors such as magnetometers are used to measure magnetic fields or changes in magnetic fields. In the detection of a magnetic object, the magnetic signal of interest may be relatively small compared to the ambient field of the earth and accordingly to obtain higher sensitivity many magnetometers are of the type which provide an output signal indicative of only a change in the magnetic field at its location as opposed to the absolute value of it. To accomplish this, the relatively steady earth's magnetic field must be subtracted from the signal containing the magnetic field change of interest.
In one type of compensation system in which the earth's magnetic field is subtracted from the measurement, a fixed voltage representing the earth's magnetic field is subtracted from the magnetometer output so that only changes to the field are observed. This compensation system, however, requires a very stable voltage source and low drift amplifiers.
In another type of compensation scheme, a high resolution data system is utilized to record the output of the magnetometer and thereafter a fixed quantity representing the earth's magnetic field is subtracted from the data so that the resulting data is the measurement of interest. This method requires extremely high resolution in the measurement of the magnetometer output in order to obtain a resultant signal with a fair degree of accuracy.
In another widely used compensation arrangement, the magnetometer is placed within a solenoid which is connected to a current source. The current through the solenoid is adjusted until it induces a field equal and opposite to the earth's magnetic field such that the magnetometer effectively sees a zero field. The solenoid field is then held at this constant value such that the output signal of the magnetometer is indicative of only change in magnetic field due to the magnetic disturbance. Although this system is highly effective, it requires continuous power, high mechanical stability in the solenoid, and extremely high stability in the current source driving the solenoid.
The present invention allows for earth field compensation in an arrangement which eliminates the objectional features of the prior art compensation arrangements.